1. Field of the Invention
The present invention generally relates to a wireless communication system, a wireless communication device, a wireless communication method and a computer program. The present invention particularly relates to a wireless communication system, a wireless communication device, a wireless communication method and a computer program that are capable of simultaneously transmitting data to a plurality of terminals.
2. Description of the Related Art
Typical examples of wireless local area network (LAN) systems include IEEE802.11a and IEEE802.11b/g that are standardized by the Institute of Electrical and Electronic Engineers (IEEE). In the IEEE802.11a/g wireless LAN system, an orthogonal frequency division multiplexing (OFDM) system is used, which provides high frequency use efficiency and which is robust against frequency selective fading. The maximum bit rate in the IEEE802.11a/g wireless LAN system is 54 Mbps.
However, in the next generation of wireless LAN systems, a higher bit rate is desired. To address this, in the IEEE systems, IEEE802.11n that uses a multi-input multi-output (MIMO) communication system is being standardized.
In the MIMO communication system, the transmission capacity can be increased in accordance with the number of antennas, without increasing the frequency band. For example, if it is assumed that the number of antennas on the transmission side is N and the number of antennas on the receiving side is M, data transmission can be performed using streams that are spatially multiplexed between a transmitter and a receiver, and it is possible to form spatial streams corresponding to the smaller number (MIN [N, M]) of the transmission and reception antennas. In summary, the transmission capacity can be increased up to MIN [N, M] times with respect to a known transmission system that is not spatially multiplexed.
Mathematical expressions and the like that relate to the MIMO communication system are described in Japanese Patent Application Publication No. JP-A-2007-318727, Japanese Patent Application Publication No. JP-A-2007-318728 and Japanese Patent Application Publication No. JP-A-2007-318729. As described in JP-A-2007-318727, JP-A-2007-318728 and JP-A-2007-318729, the MIMO communication system estimates a channel matrix H of transmission paths using a given method, and multiplies signals by an antenna weight matrix on both the transmitter and receiver sides (in some cases, only on the receiver side), thereby achieving space multiplexing transmission.
Further, Japanese Patent Application Publication No. JP-A-2007-74318 discloses a technology that improves effective speed in a wireless network by suppressing interference within the same cell in downlink, in the next generation of wireless communication system in which a plurality of antennas are used and a plurality of mobile stations perform communication at the same time using the same frequency.